This invention relates to a method of modulating an optical beam and to apparatus suitable for use in such a method.
A method of modulating an optical beam has been proposed which comprises providing an etalon structure having an absorption edge in the vicinity of the wavelength of the optical beam and comprising material of a smaller band gap sandwiched between layers of material of a larger band gap so that the smaller band gap material forms a quantum size effect confinement region for electrons and holes, directing the optical beam through the quantum size effect confinement region of the etalon structure and applying an electrical field across the etalon structure to modulate the optical beam.
Such a method is described in papers by D. A. B. Miller et al published in Applied Physics Letters 45(1) July 1, 1984 at pages 13 to 15 and in Optical Engineering May 1987 Vol 26 No 5 at pages 368 to 372 and also in EP-A-135582. As explained in the aforementioned papers, quantum well etalon structures, that is structures where layers of a smaller band gap material sandwiched between layers of a larger band gap material are so thin that quantisation of the energy levels within the smaller band gap material occurs to form a quantum effect confinement region for electrons and holes, exhibit strong exciton effects. In particular, when an electric field is applied across the quantum well layers the absorption edge of the quantum well etalon structure is shifted sideways to longer wavelength but without destroying the strong exciton resonance as would occur in a bulk sample of the material. This effect is explained in the aforementioned papers as arising, put in simple terms, because of the confinement of the electron and hole making up an exciton within the quantum well which prevents the electron and hole of the exciton being totally torn apart from one another. Because of the resemblance of this effect to the Stark shift observed when an electric field is applied to hydrogen atoms, this effect has been termed the Quantum Confined Stark Effect (QCSE).
As described in the paper published in Optical Engineering, the QCSE provides a strong electro-absorption mechanism which enables changes in absorption which make possible optical modulators that are only micrometers thick and for which the transmission can be changed by more than a factor of two with voltages of from 5 to 10 volts.
EP-A-135582 describes devices which utilise the Quantum Confined Stark Effect. In particular, there are described various optical modulators in which an electrical potential in the form of a dc bias or an ac electrical potential with a frequency of say 100 GHz is applied via electrical contacts across a multiple quantum well etalon structure so as to modulate the absorption edge of the etalon structure. An optical beam to be modulated is directed into the etalon structure either perpendicularly of or along the quantum well layers of the etalon structure. EP-A-135582 also describes the application of the Quantum Confined Stark Effect to controlling the optical path length of a Fabry-Perot cavity by providing electrical contacts to enable an electrical potential to be applied across the cavity. The paper published in Optical Engineering describes a self electro-optic device in which optical rather than only electrical control is achieved by opto-electric feedback. Thus, the etalon structure may be formed as a photo-detecting diode which is incorporated in an external circuit so that the voltage across the diode which, because of the Quantum Confined Stark Effect, changes the absorption of the diode and thus the photocurrent through the diode, is controlled by the photocurrent. The opto-electronic feedback may be made positive or negative.